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The Last Epoch. Cosmic Structure Formation. Today we have a matter-dominated universe ruled by the force of gravity. Slight variations in density produces formation regions for stars and galaxies.
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Cosmic Structure Formation • Today we have a matter-dominated universe ruled by the force of gravity. • Slight variations in density produces formation regions for stars and galaxies. • 1st generation stars and galaxies have 74% H and 26% He by mass (just like the whole universe) • 2nd gen stars are enriched
Star Formation • As H and He accrete, gas pressure increases limiting further growth unless there is sufficient matter; maximum star mass is about 60 Msun. • The Jeans mass (≈ 105Msun) constitutes the minimum amount of mass required for star formation (corresponds to globular star cluster). • Collapses of smaller clouds are helped along by: • stochastic processes (wakes of exploding stars or two or more gas clouds merging) • compression by magnetic field of galaxy
Wrinkles in Spacetime • COBE observed the cosmic microwave background, but there was an ever so slight variation in temperature (and thus density) from point to point across the sky.
Some problems with the Standard Model of Cosmology • The model of the expanding universe works quite wellto explain: • Olber’s paradox • X, Y, and Z abundances (74%, 26%, 0%, and later) • Hubble’s relationship, v = HR • 2.725K cosmic background radiation • Five major problems remain with model: • The Flatness Problem • The Horizon Problem • The Structure Problem • Dark Matter • Dark Energy
The Flatness Problem • If shortly after the Big Bang the universe was even remotely non-flat, we would not have the relatively flat universe we observe today. • Why did the universe’s original flatness balance on a knife’s edge when so many other possibilities existed? • Flatness is the midpoint between two extremes of curvature – positive and negative.
The Horizon Problem • The cosmic microwave background is amazingly uniform but for very minute variations equal to about 0.00001 K. • How is this possible when opposite “ends” of the universe cannot “communicate”?
The Structure Problem • If the universe was once entirely homogeneous and isotropic as we would expect from an early radiant universe, why is it no longer entirely so? • What cause the wrinkles in space that we spoke about earlier?
Inflationary Cosmology • In an attempt to solve the first three problems, the suggestion was made of an inflationary period in the early universe. • Between 10-34 and 10-32 seconds after the Big Bang, the universe grew exponentially – from the size of a nucleon to approximately 85 LY. • When inflation is a part of the field equations, it solves the 3 problems mentioned earlier. (New 2014 evidence supports inflation.)
The Dark Matter Problem • The universe contains dark matter the nature of which is completely unknown. • Its presence is deduced from galactic rotation curves.
The Dark Energy Problem • Dark energy is an unknown form of energy hypothesized to permeate all of space. • It explains observations since the 1990s indicating that the universe is expanding at an accelerating rate. • The best current measurements indicate that dark energy contributes 68.3% of the total energy in the present-day observable universe.
Birth of a Star • Gravity, with a bit of help from various collisions, forms stars. • At the center of a cloud, T increases to 10 million Kelvin and the P-P cycle starts. • Gravity balanced by radiation pressure for a long period of time depending on mass.
Death of a Star • H–>He–>C–>N–>O–> • Near the end of their life spans, stars become red giants. • Three possible end states determined by the mass of the star: • White dwarf (low mass) • Neutron star (medium) • Black hole (high mass)
White Dwarfs • Produced in planetary nebula phase where atmosphere begins a run away process. • An Earth-sized core is revealed at 100,000K +
Neutron Stars • Produced during a supernova explosion resulting in a pulsar. • Protons and electrons are crushed to produce neutrons.
Black Holes • Produced in supernova explosions but neutron degeneracy cannot stop collapse. • Gravity is so strong that even light cannot escape. • Accretion disk (such as Cygnus X-1) often found in binary star systems.
Black Hole Detection • X-ray sources • Gravitational effects on binary star companion • Gravitational lensing • Gravitational ripples • Accretion disk jets